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LINQ Basic Tutorial

LINQ Generate Operator DefaultIfEmpty

If the given collection to DefaultIfEmpty() is empty, the DefaultIfEmpty() method will return a new collection with the default value.

Another overloaded method of DefaultIfEmpty() accepts a value parameter that should be used as the default value.

See the following example.

IList<string> emptyList = new List<string>();
var newList1　= emptyList.DefaultIfEmpty();　
var newList2　= emptyList.DefaultIfEmpty("None");　
Console.WriteLine("Count: {0}", newList1.Count());
Console.WriteLine("Value: {0}", newList1.ElementAt(0));
Console.WriteLine("Count: {0}", newList2.Count());
Console.WriteLine("Value: {0}", newList2.ElementAt(0));
Output:

Count:　1
Value:
Count:　1
Value: None

In the above example, emptyList.DefaultIfEmpty() returns a new string collection with a null value for one of its elements, as null is the default value for string. Another method, emptyList.DefaultIfEmpty("None"), returns a string collection with one of its elements set to "None" instead of null.

This example demonstrates how to call DefaultIfEmpty on an int collection.

IList<int> emptyList = new List<int>();
var newList1　= emptyList.DefaultIfEmpty();　
var newList2　= emptyList.DefaultIfEmpty(100);
Console.WriteLine("Count: {0}", newList1.Count());
Console.WriteLine("Value: {0}", newList1.ElementAt(0));
Console.WriteLine("Count: {0}", newList2.Count());
Console.WriteLine("Value: {0}", newList2.ElementAt(0));
Output:

Count:　1
Value: 0
Count:　1
Value:　100

The following example demonstrates the DefaultIfEmpty() method for complex type collections.

IList<Student> emptyStudentList = new List<Student>();
var newStudentList1　= studentList.DefaultIfEmpty(new Student());
　　　　　　　　　　　　　　　　　
var newStudentList2　= studentList.DefaultIfEmpty(new Student() {　
　　　　　　　　　　　　　　　　StudentID = 0,　
　　　　　　　　　　　　　　　　StudentName = ""});
Console.WriteLine("Count: {0}", newStudentList1.Count());
Console.WriteLine("Student ID: {0}", newStudentList1.ElementAt(0));
Console.WriteLine("Count: {0}", newStudentList2.Count());
Console.WriteLine("Student ID: {0}", newStudentList2).ElementAt(0).StudentID);
Output:

Count:　1
Student ID:
Count:　1
Student ID: 0
LINQ Generation Operators Empty, Range, RepeatLINQ Concatenation Operator Concat